1. Technical Field
The present invention relates to a laser ignition apparatus for ignition of an internal combustion engine that is installed in a limited installation space in, for example, a motor vehicle.
2. Description of Related Art
In recent years, various laser ignition apparatuses have been proposed for ignition of internal combustion engines that are difficult to be ignited; those engines include, for example, highly-charged engines, high-compression engines, and natural gas engines with large inner diameters of cylinders. The laser ignition apparatuses are generally configured to: (1) irradiate an excitation light generated by an excitation light source (e.g., a flash lamp or a semiconductor laser) to a laser resonator (or optical resonator) that includes a laser medium and a Q switch, thereby causing the resonator to generate a pulsed laser light that has a short pulse width and a high energy density; and (2) focusing the pulsed laser light, using an optical element (e.g., a focusing lens), to a focal point (or an ignition point) in a combustion chamber of the engine to generate a flame kernel that has a high energy density, thereby igniting the air-fuel mixture in the combustion chamber.
For example, Japanese Unexamined Patent Application Publication No. 2006-220091 (to be simply referred to as Patent Document 1 hereinafter) discloses a laser-ignited engine. The engine includes both a solid target provided on the upper surface of a piston of the engine so as to face a combustion chamber of the engine and a gas target provided in the combustion chamber. The engine also includes a controller that sets the irradiating timing of a laser beam to a predetermined timing during a start or a low-load operation of the engine.
Japanese Unexamined Patent Application Publication No. 2007-506031 (to be simply referred to as Patent Document 2 hereinafter) discloses an internal combustion engine that is equipped with a laser ignition apparatus. The laser ignition apparatus includes a pumping light source, a laser resonator that includes a solid laser crystal to produce a laser beam, a Q switch for increasing the energy density of the laser beam, at least one output mirror, and a focusing device for focusing the laser beam into a combustion chamber of the engine. In addition, Patent Document 2 has an English equivalent the publication number of which is U.S. 2007/0064746 A1.
Japanese Unexamined Patent Application Publication No. 2010-537119 (to be simply referred to as Patent Document 3 hereinafter) discloses a laser ignition apparatus that includes a laser-active solid, a combustion chamber window, and a tubular housing. The combustion chamber window is connected to the housing in a gas-tight, pressure-resistant and temperature-resistant manner. In addition, Patent Document 3 has an English equivalent the publication number of which is U.S. 2010/0263615 A1.
Moreover, as shown in FIG. 1 of Patent Document 1 and FIG. 6 of Patent Document 2, the existing laser ignition apparatuses generally have the optical elements (e.g., a focusing lens and an enlarging lens) disposed in a tubular housing (or casing), and the housing is fixed to the cylinder head of the engine by tightening a male-threaded portion of the housing into a female-threaded hole formed in the cylinder head.
Therefore, during the tightening of the male-threaded portion of the housing into the female-threaded hole of the cylinder head, torsion of the housing may be caused by the tightening torque, thereby inducing mechanical stresses in the optical elements received in the housing. Consequently, due to the mechanical stresses, the optical axes of the optical elements may be distorted, thereby causing problems such as making it difficult to focus the laser beam to a desired ignition point and resulting in variation in the reflectance of the incident light and thus in variation in the output energy. As a result, the ignition of the air-fuel mixture by the laser ignition apparatus may become unstable.
Further, in the case of the laser-ignited engine disclosed in the Patent Document 1, the pulsed laser light generated by the laser resonator, which is located outside of the housing, is first transmitted to the focusing lens via an optical fiber. Then, the focusing lens, which is arranged in the housing, focuses the pulsed laser light into the combustion chamber of the engine. In this case, since only the focusing lens and an optical window member for protecting the focusing lens are received in the housing, it is possible to simplify the structure of the housing, thereby facilitating the mounting of the housing to the cylinder head. However, on the other hand, the energy loss incurred during the transmission of the pulsed laser light via the optical fiber may be so large as to cause the ignition of the air-fuel mixture to become unstable.
In the case of the laser ignition apparatus disclosed in the Patent Document 2, the pumping diodes, which together make up the pumping light source, are arranged so as to surround the outer periphery of the columnar solid laser crystal that is included in the laser resonator. The pumping diodes irradiate the excitation light to the side surface of the solid laser crystal, thereby causing the pulsed laser light to be outputted in the direction of a longitudinal axis of the solid laser crystal. Therefore, in this case, the radial size of the laser resonator may be considerably larger than that in the case of irradiating the excitation light to that end face of the solid laser crystal which is on the proximal side (i.e., on the opposite side to the combustion chamber) in the direction of the longitudinal axis of the solid laser crystal.
In addition, to cool the solid laser crystal, a cooling device, which is comprised of Peltier cooling elements and two liquid cooling circulation systems, is further provided around the pumping diodes. As a result, as shown in FIG. 1 of Patent Document 2, at the proximal-side end of the elongated tubular housing, there is formed a solid laser unit that has a very large radial size. Accordingly, when there is only a limited installation space above the cylinder head, it may be difficult to mount the laser ignition apparatus to the cylinder head.
In particular, in recent years, there is a tendency of minimizing the diameters of plug holes (i.e., the through-holes formed in cylinder heads of engines for mounting spark plugs to the cylinder heads). Thus, there is also a demand for minimizing the sizes of spark plugs.
Accordingly, it is also required to minimize the sizes of laser ignition apparatuses. However, with the configuration of the laser ignition apparatus disclosed in Patent Document 2, it is difficult to meet the above requirement.
In addition, with the large solid laser unit formed at the proximal end of the elongated tubular hosing, when an external vibration or shock is transmitted to the laser ignition apparatus, the moment of inertia loaded on the housing will be large. Consequently, the optical axis connecting the solid laser unit and the focusing lens may be distorted, thereby making it impossible to focus the pulsed laser light to a suitable ignition point in the combustion chamber and thus causing the ignition of the air-fuel mixture to become unstable.
In the case of the laser ignition apparatus disclosed in Patent Document 3, the tubular housing has both a laser resonator and a focusing lens received therein. The laser resonator is comprised of an input mirror, the laser-active solid, a Q switch and an output mirror. On the other hand, the pumping light source (or excitation light source) is located outside of the housing. When the pumping light (or excitation light) generated by the pumping light source is irradiated to the laser resonator from the proximal side, the temperature of the laser-active solid will be increased, thereby varying the cycle of the pulsed laser light generated by the laser resonator. In addition, due to the difference in coefficient of thermal expansion between the housing and the laser-active solid, tensile stress or compressive stress will be induced in the laser-active solid, thereby distorting the optical axis of the pulsed laser light. As a result, it may become impossible to focus the pulsed laser light to a suitable ignition point in the combustion chamber, causing the ignition of the air-fuel mixture to become unstable.
Further, as shown in FIG. 2 of Patent Document 3, to separate all the components received in the housing from the combustion chamber, the combustion chamber window, which is made of a heat-resistant glass, is joined to a distal-side end face (i.e., a combustion chamber-side end face) of the metallic housing by, for example, soldering or a ceramic adhesive. However, the joint formed between the combustion chamber window and the housing is located inside the combustion chamber and thus directly exposed to the air-fuel mixture whose pressure and temperature change greatly. Therefore, even if the differences in coefficient of thermal expansion between the housing, the combustion chamber window and the joining material are made small and a surface-active material is used therebetween, it is still possible for the joining material to peel off from the housing and the combustion chamber window due to age-related deterioration. Consequently, the combustion chamber window may be detached from the housing to fall into the combustion chamber, thereby damaging the engine. That is to say, the laser ignition apparatus may lack reliability.
In addition, in another embodiment of Patent Document 3, the housing has a two-part structure consisting of an inner shell and an outer shell. The outer shell has a projection formed at a distal-side end thereof. The combustion chamber window, which is substantially flat plate-shaped, has its outer peripheral portion retained between the inner shell and the projection of the outer shell (see FIG. 3 of Patent Document 3). Consequently, the combustion chamber window can be prevented from being detached from the housing and thus from falling into the combustion chamber. However, in this case, the combustion chamber window is inevitably recessed from the projection of the outer shell of the housing toward the proximal side (i.e., in the axial direction away from the combustion chamber), forming a step between the combustion chamber window and the projection.
Consequently, when the flow of air-fuel mixture or fuel spray in the combustion chamber passes through the outer surface of the combustion chamber window, a vortex flow may be generated in the vicinity of the step formed between the combustion chamber window and the projection of the housing, causing unburned fuel or soot included in the flow to deposit on the inside of the step. Further, the deposit of the unburned fuel or soot may gradually expand from the outer periphery to the center of the outer surface of the combustion chamber window, causing the optical axis of the pulsed laser light to be distorted and thereby making it impossible to perform normal ignition of the air-fuel mixture.
In addition, an ignition failure due to the deposit of unburned fuel or soot on the outer surface of an optical window member may be caused not only in the laser ignition apparatus disclosed in Patent Document 3, but also in other existing laser ignition apparatuses.